Phase-contrast velocity mapping
Quantifying flow in cardiac MRI calls on the same principles as those set out for phase-contrast MR angiography. Phase-contrast flow imagery gives access to quantifying blood velocities and flows.
Phase-contrast acquisition comprises sequences with and without encoding of the flows that produce the images in magnitude (“anatomical” aspect of flows) and in phase (“quantitative” aspect: flow direction and velocity). A suitable encoding speed must be chosen beforehand to avoid an aliasing source of errors in high speed measurement.
Cardiac gating of these sequences is retrospective, with continuous gradient echo acquisition and phase encoding changes for each R wave.
To quantify the flows, the acquisition plane must be perpendicular to the vessel of interest (through-plane). To qualitatively visualize the flow lines, the slice plane must follow the flow axis (in-plane). The number and orientation of the velocity-encoding gradients are adapted to the direction of the flow considered and the type of image (through-plane or in-plane).
Accurate flow quantification demands:
- Sufficient spatial resolution to accurately measure the diameter of the vessel
- Precise temporal resolution covering the whole cardiac cycle
- An angle close to 90° between the slice plane and the flow axis
- An adapted encoding speed, slightly above the maximum velocity of the studied flow.